In recent years, the spread of various electric vehicles has been anticipated for solving environmental and energy problems. For an on-vehicle power source such as a motor driving power source, which is the key for practical application of such electric vehicles, the development of lithium ion secondary batteries has been extensively conducted. However, for widely spreading the battery as the on-vehicle power source, the battery needs to have higher performance and be less expensive. Moreover, the mileage per charge of an electric vehicle needs to be as long as that of a gasoline-powered vehicle. Thus, the higher energy battery has been desired.
For increasing the energy density of the battery, it is necessary to increase the amount of electricity that can be stored in a positive electrode and a negative electrode per unit mass. As a positive electrode material (active material for a positive electrode) that can meet this demand, a so-called solid-solution positive electrode has been examined. Above all, a solid solution including electrochemically inactive layered Li2MnO3 and electrochemically active layered LiAO2 (A represents a transition metal such as Co or Ni) has been expected as a candidate for a high-capacity positive electrode material that can exhibit a high electric capacity of more than 200 mAh/g (see, for example, Patent Document 1). Moreover, as a positive electrode material, a general formula LipNxMyOzPbFa has been known (see Patent Document 2), wherein N represents at least one element selected from the group consisting of Co, Mn, and Ni, and M represents at least one element selected from the group consisting of transition metal elements other than Co, Mn, and Ni, and alkaline earth metal elements, which contains at least Al.